1. Field of the Invention
The present invention relates to an ultrasonic nebulizer (atomizer) and in particular to a nebulizer of the type having an output controllable dependent on the level of liquid available for nebulization.
2. Description of the Prior Art
Ultrasonic nebulizers are devices which utilize a source of ultrasound, such as for example a piezoelectric crystal oscillator, acoustically coupled to a liquid in a nebulizing chamber in order to generate an aerosol of small liquid droplets in a space above the liquid boundary. The generated aerosol may be used for any desired purpose such as humidification or medication. Such nebulizers are often used as a component in a breathing circuit of a mechanical ventilator, where they are employed in the delivery of controlled doses of anaesthetic or other additive into a breathing gas for supply to a patient.
It is important, particularly in the medical field, to be able to monitor the level of liquid in the nebulizing chamber. This may be for example, in order to maintain a supply of liquid throughout mechanical ventilation or to monitor the dosage of liquid delivered into the breathing gas.
One known ultrasonic nebulizer which is provided with a liquid level indicator is disclosed in U.S. Pat. No. 3,839,651. This nebulizer uses a temperature sensitive resistance element which is thermally coupled to the liquid within the nebulizing chamber. The current in an electrical circuit containing this element is dependent on the amount of liquid within the chamber and is used to decrease power supplied to the oscillator and to provide a visible indication when the liquid level falls to a predetermined minimum. One problem with such a level indicator is that it is relatively insensitive to small changes in liquid level which are likely to occur between successive, or closely spaced, inspiration periods of a patient breathing cycle.
An object of the present invention is to provide an ultrasonic nebulizer having a level indicator capable of sensing such small changes.
The above object is achieved in accordance with the principles of the present invention in a nebulizer having a nebulization chamber containing a liquid to be nebulized the liquid having an upper boundary within the chamber, and an ultrasonic nebulization source which is acoustically coupled to the liquid to introduce ultrasound into the liquid to nebulizer the liquid, and wherein the ultrasonic nebulization source is operated to emit ultrasound with a variable amplitude so as to provide a measurement period during which no nebulization occurs, and wherein the nebulizer has a sonar device which, during the measurement period, measures a time interval between emission of an acoustic pulse toward said liquid boundary and detection of a component of the emitted acoustic pulse reflected from the boundary so as to produce an output signal dependent on this measured time interval which is indicative of a location of the upper boundary of the liquid within the nebulization chamber.
By controlling the amplitude of the nebulization source to provide periods where no nebulization occurs, possibly by providing periods of zero amplitude output, a sonar device which employs echo ranging techniques may be used to measure the location of the upper boundary of the liquid within the nebulization chamber. This provides a relatively sensitive arrangement for identifying changes in the location of the liquid boundary from which, for example, the amount of liquid within the nebulization chamber may be calculated.
Preferably, a single piezoelectric crystal is employed as both the nebulization source and as the sonar device. This allows existing nebulizing chambers and sources to be used with only modifications to the electronic circuitry used to control the crystal being necessary. Moreover, by using only one crystal, a major component cost saving is achieved compared with employing separate sonar and nebulization sources.
A difference forming circuit is used to enable differences in the location of the liquid boundary to be determined. The determined difference, for example, may be used to monitor the amount of liquid nebulizer between measurement periods or to monitor the effect of different known crystal driving currents on the liquid boundary during a single measurement period. Both of these monitoring modes then may be employed to calibrate the nebulization source and to control the amplitude or duration of the ultrasonic output from the source to, for example, more reliably provide a required amount of nebulization or to remove power from the source if a minimum liquid level is reached.
Particularly useful is the latter mode of monitoring since a calibration of the output of the nebulization source may be made before generating any nebulized liquid.